Simplified cable selector
Read the Appendix C device rating and voltage drop tables.
Reads AS/NZS 3000:2018. Tables checked 26 Sept 2026.
Try a job 6
Each job fills in example values. Replace them with your values.
- Match a breaker to a 2.5 mm² cableThe largest breaker for a 2.5 mm² cable partly in ceiling insulation over 30 m.
- Match a breaker to lighting cableThe largest breaker for 1.5 mm² over 35 m.
- Match a breaker to 4 mm²The largest breaker for 4 mm² over 25 m.
- Match a breaker to oven cableThe largest breaker for 6 mm² over 15 m.
- Check a three-phase 10 mm² circuitThe largest breaker for 10 mm², three phase, over 40 m.
- Match a breaker in full insulationThe largest breaker for 2.5 mm² fully in insulation.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Cable size
- 2.5 mm²
- Phases
- Single phase, 230 V
- Installation condition
- Unenclosed in air
- Design current
- 20 A
- Route length
- 25 m
- Voltage drop limit
- 5 %
Maximum device rating
25A
- Voltage drop
- 3.91%
- Ampere metres
- 500
- Size for the drop limit
- 2.5mm²
This result is a design aid. Verify it against the current standard.
Calculation steps
- Maximum device rating InResult25A
AS/NZS 3000:2018 Tables C6 and C7
- Ampere metres500
AS/NZS 3000:2018 Table C8
- Ampere metres per percent for this size128
AS/NZS 3000:2018 Table C8
- Voltage drop3.91%
AS/NZS 3000:2018 Table C8
- Ampere metres per percent needed for the limit100
AS/NZS 3000:2018 Table C8
- Smallest size for the voltage drop limit2.5mm²
AS/NZS 3000:2018 Table C8
About this calculator
How this is calculated
The tool reads the simplified tables of AS/NZS 3000 Appendix C. They cover copper cable from 1 mm² to 25 mm². They give the largest device rating and the voltage drop in one pass.
- Maximum device rating. The tool reads the rating for your size, phases and installation condition from Tables C6 and C7.
- Ampere metres. It multiplies the load current by the one-way length.
- Ampere metres per percent. It reads the value for your size from Table C8.
- Voltage drop. It divides the ampere metres by that value. The result is the drop in percent.
- Size for the limit. It divides the ampere metres by your limit. It then picks the smallest size from Table C8 that meets the figure.
| Step | Table or clause | What it decides |
|---|---|---|
| Device rating | AS/NZS 3000 Tables C6 and C7 | The largest device for the size and condition |
| Drop and size | AS/NZS 3000 Table C8 | The drop for the run and the size for your limit |
Worked example
- 2.5 mm² on one phase over 25 m at 20 A. The cable is in unenclosed air. The maximum device rating is 25 A. The run is 500 ampere metres. The drop is 3.91 %, inside the 5 % limit. The size for the limit is 2.5 mm².
Limits
The tables assume one circuit on its own. Where circuits are grouped, use the cable size calculator. The tool does not apply correction factors. It does not read AS/NZS 3008.1 tables. It stops at 25 mm².
See also: Cable size for a 32 A single-phase run of 25 m, AS/NZS 3000 Clause 3.6.2 voltage drop limit and Voltage drop basics.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What does the simplified selector do?
The tool reads AS/NZS 3000:2018 Tables C6 and C7 for the largest protective device a cable from 1 to 25 square millimetres may carry. It then reads Table C8 for the voltage drop over the run and the smallest size that meets the limit.
When should I use the full cable size tool instead?
Use the cable size tool when circuits are grouped, when the ambient temperature is not 30 degrees, when the cable is not PVC two-core or four-core and earth, or when the size is above 25 square millimetres. Appendix C assumes one circuit on its own.
How is the voltage drop worked out?
Multiply the design current by the route length to get ampere metres. Divide that by the ampere metres per percent for the size in Table C8. The answer is the drop as a percent of 230 volts single phase or 400 volts three phase.
Which installation condition should I pick?
Match the way the cable sits. A cable clipped in free air reads as unenclosed. A cable in a conduit, a duct or a wall cavity reads as enclosed in air. A buried run reads as enclosed in ground. Insulation around the cable reads as partly or completely surrounded. Note 3 to Table C6 gives the full mapping.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Table C6, C7Simplified protective device selection for 1 to 25 mm2 cables
- AS/NZS 3000:2018 Table C8Simplified voltage drop, ampere metres per percent
Every clause and table, with how each was verified
Tables last checked on 26 Sept 2026.
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